Positioning method

ABSTRACT

A reception environment is determined to be a multipath environment when the difference (positioning altitude difference) between the maximum value (maximum altitude) and the minimum value (minimum altitude) of the altitudes of candidate present positions P of respective satellite sets exceeds a given threshold value (e.g., 200 m), and is determined to be an open-sky environment when the positioning altitude difference is equal to or less than the given threshold value. When the reception environment is the open-sky environment, an evaluation point E of each satellite set is calculated using a known evaluation method based on the number of satellites, a PDOP value, and the like. When the reception environment is the multipath environment, the evaluation point E of each satellite set is calculated in the same manner as in the open-sky environment, and a change amount ΔE corresponding to the difference (altitude difference) between the altitude corresponding to the preceding located position acquired from an altitude table and the altitude of the candidate present position of the target satellite set is subtracted from the evaluation point E.

Japanese Patent Application No. 2007-112609 filed on Apr. 23, 2007 andJapanese Patent Application No. 2008-37117 filed on Feb. 19, 2008, arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a positioning method, a storage medium,a positioning device, and an electronic instrument.

The global positioning system (GPS) is widely known as a satellitepositioning system, and is utilized for a car navigation system and thelike. In the GPS, GPS satellite signals are respectively transmittedfrom a plurality of GPS satellites which orbit the earth, and a GPSreceiver calculates (locates) its present position based on the receivedGPS satellite signals.

A GPS satellite signal affected by a multipath or the like may beincluded in the acquired GPS satellite signals. The term “multipath”refers to a phenomenon in which an indirect wave reflected or diffractedby a building or topography is superimposed on a direct wave from a GPSsatellite so that the GPS receiver receives identical radio wavesthrough multiple paths. Such a reception environment is referred to as amultipath environment. The present position of the GPS receiver may notbe accurately calculated (located) when using a GPS satellite signalaffected by a multipath. To deal with this problem, a method has beenknown which improves positioning accuracy utilizing an altitude table(urban altitude information) in a multipath area in which a number ofbuildings such as high buildings are present (e.g., the center ofTokyo). The altitude table is a data table in which a central urban areahaving a small difference in altitude is divided into a predeterminednumber of (i.e., nine) mesh areas and the altitude corresponding to eachmesh area is specified (see JP-A-2006-177783, for example).

The GPS receiver generally calculates its position as follows.Specifically, the GPS receiver selects satellite sets (i.e.,combinations of four or more GPS satellites) based on the acquired GPSsatellite signals, and performs positioning calculations correspondingto the respective satellite sets to calculate candidate presentpositions. The GPS receiver evaluates the positioning accuracy of thecandidate present positions calculated corresponding to the respectivesatellite sets based on an index such as a position dilution ofprecision (PDOP), and determines the candidate present position havingthe highest evaluation result to be the present positioning result.

However, the positioning accuracy of each satellite set does not differto a large extent in a reception environment with high overallpositioning accuracy (e.g., open-sky environment). On the other hand,GPS satellite signals are affected by a multipath to a different extentin a reception environment with poor positioning accuracy (e.g.,multipath environment). As a result, the positioning accuracy of eachsatellite set may differ to a large extent. In this case, it isimportant to appropriately determine a satellite set with highpositioning accuracy. According to related-art technology, the candidatepresent position of each satellite set is evaluated using an evaluationmethod similar to that used for a reception environment with highpositioning accuracy (e.g., open-sky environment).

SUMMARY

According to one aspect of the invention, there is provided apositioning method comprising:

selecting satellite sets, each of the satellite sets being a combinationof satellites used for present positioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

determining a reception environment based on a dispersion of the presentposition altitudes of the respective satellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using an evaluation method corresponding tothe reception environment; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

According to another aspect of the invention, there is provided apositioning method comprising:

selecting satellite sets, each of the satellite sets being a combinationof satellites used for present positioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using a given evaluation method that adjustsan evaluation result based on a difference between the present positionaltitude of a target satellite set among the satellite sets and a givenreference altitude; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an internal configuration diagram of a portable telephone.

FIG. 2 is a graph of a change amount calculation equation f.

FIGS. 3A and 3B show experimental results indicating the relationshipbetween a reception environment and a positioning error.

FIG. 4 is a configuration diagram of a ROM.

FIG. 5 shows a data configuration example of an altitude table.

FIG. 6 shows a data configuration example of a reception environmentdetermination condition table.

FIG. 7 shows a data configuration example of a calculation equationselection condition table.

FIG. 8 is a configuration diagram of a RAM.

FIG. 9 shows a data configuration example of satellite position data.

FIG. 10 shows a data configuration example of satellite set data.

FIG. 11 shows a data configuration example of positioning result data.

FIG. 12 is a flowchart of a baseband process.

DETAILED DESCRIPTION OF THE EMBODIMENT

Several embodiments of the invention may enable the candidate presentposition of each satellite set to be appropriately evaluatedcorresponding to the reception environment.

According to one embodiment of the invention, there is provided apositioning method when receiving satellite signals transmitted frompositioning satellites and repeating present position positioningcalculations based on the received satellite signals, the positioningmethod comprising:

selecting satellite sets based on the received satellite signals, eachof the satellite sets being a combination of satellites used for presentpositioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using thesatellite signals from the satellites included in the respectivesatellite sets;

determining a reception environment based on a dispersion of the presentposition altitudes of the respective satellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using an evaluation method corresponding tothe reception environment; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

According to another embodiment of the invention, there is provided apositioning device that receives satellite signals transmitted frompositioning satellites and performs present position positioningcalculations based on the received satellite signals, the positioningdevice comprising:

a satellite set selection section that selects satellite sets based onthe received satellite signals, each of the satellite sets being acombination of satellites used for present positioning calculations;

an individual satellite set present position calculation section thatcalculates candidate present positions including a present positionaltitude corresponding to the respective satellite sets using thesatellite signals from the satellites included in the respectivesatellite sets;

an environment determination section that determines a receptionenvironment based on a dispersion of the present position altitudes ofthe respective satellite sets;

an evaluation section that evaluates the candidate present positionscorresponding to the respective satellite sets using an evaluationmethod corresponding to the reception environment; and

a present position determination section that selects a candidatepresent position from the candidate present positions corresponding tothe respective satellite sets based on the evaluation results, anddetermines the selected candidate present position to be a presentlocated position.

According to the above configuration, the candidate present positionincluding the present position altitude is calculated corresponding toeach satellite set selected based on the received satellite signals, andthe reception environment is determined based on the dispersion of thecalculated present position altitude of each satellite set. Thecandidate present position corresponding to each satellite set isevaluated using the evaluation method corresponding to the determinedreception environment, and the candidate present position selected basedon the evaluation results is determined to be the present locatedposition.

The term “dispersion” refers to an index that indicates the degree(variation) of distribution, and is indicated by the difference betweenthe maximum value and the minimum value, a standard deviation, avariance, or the like. Specifically, when the variation in the presentposition altitude corresponding to each satellite set is large, thereception environment is likely to be a reception environment (e.g.,multipath environment) with a large positioning error. On the otherhand, when the variation in the present position altitude correspondingto each satellite set is small, the reception environment is likely tobe a reception environment (e.g., open-sky environment) with a smallpositioning error. Therefore, the reception environment can bedetermined based on the dispersion of the calculated present positionaltitude corresponding to each satellite set, and the candidate presentposition corresponding to each satellite set can be appropriatelyevaluated using the evaluation method corresponding to the receptionenvironment. As a result, the candidate present position determined tobe the located position can be appropriately selected from the candidatepresent positions corresponding to the respective satellite sets.

In the positioning method,

the determining of the reception environment may include determining thedispersion based on a difference between a maximum altitude and aminimum altitude of the present position altitudes of the respectivesatellite sets.

In the positioning device,

the environment determination section may determine the receptionenvironment while determining the dispersion based on a differencebetween a maximum altitude and a minimum altitude of the presentposition altitudes of the respective satellite sets.

According to the above configuration, the reception environment isdetermined by determining the dispersion based on a difference betweenthe maximum altitude and the minimum altitude of the calculated presentposition altitudes of the respective satellite sets. For example, thepositioning error of each satellite set is large in a receptionenvironment (e.g., multipath environment) with low positioning accuracy.Therefore, the variation in the calculated present position altitude islarge. On the other hand, the positioning error of each satellite set issmall in a reception environment (e.g., open-sky environment) with highpositioning accuracy. Therefore, the variation in the calculated presentposition altitude is small. Therefore, the reception environment (e.g.,positioning accuracy is high or low) can be determined based on thedifference between the maximum altitude and the minimum altitude of thecalculated present position altitudes.

In the positioning method,

the determining of the reception environment may include determining areception environment from a plurality of reception environmentsincluding a multipath environment; and

the evaluating of the candidate present positions may include adjustingthe evaluation results of the candidate present positions correspondingto the respective satellite sets based on a difference between thepresent position altitude of a target satellite set among the satellitesets and a given reference altitude, and increasing an adjustment amountwhen the reception environment has been determined to be the multipathenvironment as compared with a case where the reception environment hasbeen determined to be a reception environment other than the multipathenvironment.

In the positioning device,

the environment determination section may determine a receptionenvironment from a plurality of reception environments including amultipath environment; and

the evaluation section may adjust the evaluation results of thecandidate present positions corresponding to the respective satellitesets based on a difference between the present position altitude of atarget satellite set among the satellite sets and a given referencealtitude, and may increase an adjustment amount when the environmentdetermination section has determined that the reception environment isthe multipath environment as compared with a case where the environmentdetermination section has determined that the reception environment is areception environment other than the multipath environment.

According to the above configuration, the reception environment isdetermined from a plurality of reception environments including themultipath environment. The evaluation results of the candidate presentpositions corresponding to the respective satellite sets are adjustedbased on the difference between the present position altitude and thegiven reference altitude, and the adjustment amount is increased whenthe reception environment has been determined to be the multipathenvironment as compared with the case where the reception environmenthas been determined to be a reception environment other than themultipath environment. Therefore, the candidate present positions can beappropriately evaluated corresponding to the reception environment bydecreasing the evaluation results in the multipath environment to alarge extent when the positioning accuracy of the multipath environmentis lower than that of other reception environments, for example.

According to another embodiment of the invention, there is provided apositioning method when receiving satellite signals transmitted frompositioning satellites and repeating present position positioningcalculations based on the received satellite signals, the positioningmethod comprising:

selecting satellite sets based on the satellite signals, each of thesatellite sets being a combination of satellites used for presentpositioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using thesatellite signals from the satellites included in the respectivesatellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using a given evaluation method that adjustsan evaluation result based on a difference between the present positionaltitude of a target satellite set among the satellite sets and a givenreference altitude; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

According to another embodiment of the invention, there is provided apositioning device that receives satellite signals transmitted frompositioning satellites and repeats present position positioningcalculations based on the received satellite signals, the positioningdevice comprising:

a satellite set selection section that selects satellite sets based onthe satellite signals, each of the satellite sets being a combination ofsatellites used for present positioning calculations;

an individual satellite set present position calculation section thatcalculates candidate present positions including a present positionaltitude corresponding to the respective satellite sets using thesatellite signals from the satellites included in the respectivesatellite sets;

an evaluation section that evaluates the candidate present positionscorresponding to the respective satellite sets using a given evaluationmethod that adjusts an evaluation result based on a difference betweenthe present position altitude of a target satellite set among thesatellite sets and a given reference altitude; and

a present position determination section that selects a candidatepresent position from the candidate present positions corresponding tothe respective satellite sets based on the evaluation results, anddetermines the selected candidate present position to be a presentlocated position.

According to the above configuration, the candidate present positionincluding the present position altitude is calculated corresponding toeach satellite set selected based on the received satellite signals, andthe candidate present position selected based on the evaluation resultsof the calculated candidate present positions corresponding to thesatellite sets is determined to be the present located position. Thecandidate present position corresponding to each satellite set isevaluated using the given evaluation method that adjusts the evaluationresult corresponding to the difference between the present positionaltitude of the satellite set and the given reference altitude.Therefore, the candidate present positions can be appropriatelyevaluated corresponding to the reception environment by decreasing theevaluation results to a large extent in a reception environment (e.g.,multipath environment) with positioning accuracy lower than that ofother reception environments, for example.

In the positioning method,

the evaluating of the candidate present positions may include evaluatingthe candidate present positions using an altitude of a located positiondetermined by preceding positioning calculations as the given referencealtitude.

In the positioning device,

the evaluation section may evaluate the candidate present positionsusing an altitude of a located position determined by precedingpositioning calculations as the given reference altitude.

According to the above configuration, the candidate present positioncorresponding to each satellite set is evaluated using the altitude ofthe located position determined by the preceding positioningcalculations as the reference altitude. For example, when repeatedlylocating the present position at given time intervals (e.g., intervalsof one second), the positioning accuracy of the target satellite set canbe determined based on the difference between the present positionaltitude of each satellite set and the altitude of the preceding locatedposition. Specifically, the larger the difference in altitude, the lowerthe positioning accuracy is.

In the positioning method,

the evaluating of the candidate present positions may include acquiringan altitude corresponding to the located position determined by thepreceding positioning calculations from given altimeter data thatdefines altitudes of each area, and evaluating the candidate presentpositions using the acquired altitude as the given reference altitude.

In the positioning device,

the evaluation section may acquire an altitude corresponding to thelocated position determined by the preceding positioning calculationsfrom given altimeter data that defines altitudes of each area, and mayevaluate the candidate present positions using the acquired altitude asthe given reference altitude.

According to the above configuration, the candidate present positioncorresponding to each satellite set is evaluated using the altitudecorresponding to the located position determined by the precedingpositioning calculations and acquired from the given altimeter data thatdefines the altitude of each area as the reference altitude. Forexample, when repeatedly locating the present position at given timeintervals (e.g., intervals of one second), the positioning accuracy ofthe target satellite set can be determined based on the differencebetween the present position altitude of each satellite set and thealtitude corresponding to the preceding located position. Specifically,the larger the difference in altitude, the lower the positioningaccuracy is. Since the located position determined by positioningcalculations is not necessarily an accurate value, the positioningaccuracy can be more accurately determined using the altitude acquiredfrom the altitude data.

According to another embodiment of the invention, there is provided apositioning method comprising:

selecting satellite sets, each of the satellite sets being a combinationof satellites used for present positioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

determining a reception environment based on a dispersion of the presentposition altitudes of the respective satellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using an evaluation method corresponding tothe reception environment; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

According to another embodiment of the invention, there is provided apositioning method comprising:

selecting satellite sets, each of the satellite sets being a combinationof satellites used for present positioning calculations;

calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

evaluating the candidate present positions corresponding to therespective satellite sets using a given evaluation method that adjustsan evaluation result based on a difference between the present positionaltitude of a target satellite set among the satellite sets and a givenreference altitude; and

selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.

According to another embodiment of the invention, there is provided apositioning device comprising:

a satellite set selection section that selects satellite sets, each ofthe satellite sets being a combination of satellites used for presentpositioning calculations;

an individual satellite set present position calculation section thatcalculates candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

an environment determination section that determines a receptionenvironment based on a dispersion of the present position altitudes ofthe respective satellite sets;

an evaluation section that evaluates the candidate present positionscorresponding to the respective satellite sets using an evaluationmethod corresponding to the reception environment; and

a present position determination section that selects a candidatepresent position from the candidate present positions corresponding tothe respective satellite sets based on the evaluation results, anddetermines the selected candidate present position to be a presentlocated position.

According to another embodiment of the invention, there is provided apositioning device comprising:

a satellite set selection section that selects satellite sets, each ofthe satellite sets being a combination of satellites used for presentpositioning calculations;

an individual satellite set present position calculation section thatcalculates candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;

an evaluation section that evaluates the candidate present positionscorresponding to the respective satellite sets using a given evaluationmethod that adjusts an evaluation result based on a difference betweenthe present position altitude of a target satellite set among thesatellite sets and a given reference altitude; and

a present position determination section that selects a candidatepresent position from the candidate present positions corresponding tothe respective satellite sets based on the evaluation results, anddetermines the selected candidate present position to be a presentlocated position.

Another embodiment of the invention relates to a computer-readablestorage medium storing a program that causes a computer to execute theabove positioning method, the computer being included in a positioningdevice that receives satellite signals transmitted from positioningsatellites and locates a present position of the positioning devicebased on the received satellite signals.

The term “storage medium” used herein refers to a storage medium (e.g.,hard disk, CD-ROM, DVD, memory card, or IC memory) from whichinformation stored therein can be read by a computer.

A further embodiment of the invention relates to an electronicinstrument comprising the above positioning device.

Preferred embodiments of the invention are described in detail belowwith reference to the drawings.

The following embodiments illustrate specific preferred examples of theinvention and are provided with various technologically preferredlimitations. Note that the scope of the invention is not limited to thefollowing embodiments unless there is a description limiting theinvention.

An embodiment in which the invention is applied to a portable telephonehaving a GPS positioning function is described below with reference tothe drawings.

Configuration

FIG. 1 is a block diagram showing the internal configuration of aportable telephone 1 according to this embodiment. As shown in FIG. 1,the portable telephone 1 includes a GPS antenna 10, a GPS receiversection 20 (positioning device), a host central processing unit (CPU)40, an operation section 41, a display section 42, a read-only memory(ROM) 43, a random access memory (RAM) 44, a portable telephone antenna50, and a portable telephone wireless communication circuit section 60.

The GPS antenna 10 is an antenna which receives an RF signal including aGPS satellite signal transmitted from a GPS satellite, and outputs thereceived RF signal.

The GPS receiver section 20 acquires/extracts the GPS satellite signalfrom the RF signal received by the GPS antenna 10, and calculates thepresent position of the portable telephone 1 by performing positioningcalculations based on a navigation message and the like extracted fromthe GPS satellite signal. The GPS receiver section 20 includes a radiofrequency (RF) receiver circuit section 21, an oscillation circuit 22,and a baseband process circuit section 30. The RF receiver circuitsection 21 and the baseband process circuit section 30 may be producedas different large scale integrated (LSI) circuits, or may be producedin one chip.

The oscillation circuit 22 is a crystal oscillator or the like whichgenerates and outputs an oscillation signal having a given oscillationfrequency.

The RF receiver circuit section 21 multiplies the RF signal input fromthe GPS antenna 10 by a signal obtained by dividing or multiplying thefrequency of the oscillation signal input from the oscillation circuit22 to down-convert the RF signal into an intermediate-frequency signal(hereinafter referred to as “IF signal”). The RF receiver circuitsection 21 amplifies the IF signal, converts the amplified signal into adigital signal using an A/D converter, and outputs the digital signal,for example.

The baseband process circuit section 30 is a circuit section whichacquires/tracks the GPS satellite signal from the IF signal input fromthe RF receiver circuit section 21, and performs pseudo-rangecalculations, positioning calculations, and the like based on anavigation message, time information, and the like extracted by decodingdata.

Specifically, the baseband process circuit section 30 acquires the GPSsatellite signal based on the input IF signal. The baseband processcircuit section 30 acquires the GPS satellite signal by extracting theGPS satellite signal from the IF signal by performing a correlationprocess on the IF signal. Specifically, the baseband process circuitsection 30 performs a coherent process which calculates the correlationbetween the IF signal and a pseudo-generated C/A code replica (codereplica) using FFT calculations, and an incoherent process whichintegrates the correlation values (i.e., the results of the coherentprocess) to calculate an integrated correlation value. As a result, thephases of the C/A code and a carrier frequency contained in the GPSsatellite signal are obtained.

After acquiring the GPS satellite signal, the baseband process circuitsection 30 tracks the acquired GPS satellite signal. The basebandprocess circuit section 30 tracks the GPS satellite signals bysynchronously holding a plurality of acquired GPS satellite signals inparallel. For example, the baseband process circuit section 30 performsa code loop which is implemented by a delay locked loop (DLL) and tracksthe phase of the C/A code, and a carrier loop which is implemented by aphase locked loop (PLL) and tracks the phase of the carrier frequency.The baseband process circuit section 30 extracts the navigation messageby decoding data contained in each GPS satellite signal which has beentracked, and performs pseudo-range calculations, positioningcalculations, and the like to locate the present position.

The baseband process circuit section 30 includes a CPU 31, a ROM 32, anda RAM 33. The baseband process circuit section 30 also includes variouscircuits such as a C/A code replica generation circuit, a correlationcalculation circuit, and a data decoder circuit.

The CPU 31 controls each section of the baseband process circuit section30 and the RF receiver circuit section 21, and performs variouscalculations including a baseband process described later.

In the baseband process, the CPU 31 decodes the navigation messageincluded in the acquired and tracked GPS satellite signal, andcalculates the present position of the portable telephone 1 based onorbit information and time information of the GPS satellite included inthe decoded navigation message. Specifically, the CPU 31 calculates theposition of each GPS satellite and the pseudo-range between the GPSreceiver and each GPS satellite when the GPS satellite has transmittedthe GPS satellite signal based on the difference between the time atwhich each GPS satellite has transmitted the GPS satellite signal andthe time at which the GPS receiver has received the GPS satellitesignal. The CPU 31 calculates the present position by solvingsimultaneous equations in which the present position and a time errorbetween the GPS satellite and the GPS receiver are unknown quantities.The CPU 31 can calculate the present position by receiving GPS satellitesignals from at least four GPS satellites. This is because thecoordinate values (x,y,z) of the present position (i.e.,three-dimensional position) and a time error T between the GPS satelliteand the GPS receiver are used as unknown quantities.

Specifically, the CPU 31 selects satellite sets (e.g., combinations offour or more GPS satellites) based on the acquired GPS satellitesignals. For example, when eight GPS satellite signals have beenacquired, the CPU 31 selects 163 (=₈C₈+₈C₇+₈C₆+₈C₅+₈C₄) satellite sets.The CPU 31 then performs positioning calculations corresponding to eachselected satellite set using a least-square method or the like tocalculate candidate present positions P of the portable telephone 1. Theterm “candidate present position P” refers to a three-dimensionalposition represented by latitude, longitude, and altitude, for example.Note that about several tens of satellite sets may be extracted from thesatellite sets generated based on the acquired GPS satellite signals,and the extracted satellite sets may be determined to be the targetsatellite sets.

The CPU 31 determines the reception environment based on the calculatedcandidate present position of each satellite set. In this example, theCPU 31 determines that the reception environment to be an open-skyenvironment or a multipath environment. Specifically, the CPU 31extracts the maximum value (maximum altitude) and the minimum value(minimum altitude) of the altitudes of the candidate present positionscorresponding to the respective satellite sets, and calculates apositioning altitude difference which is the difference between themaximum value and the minimum value thus extracted. The CPU 31determines the reception environment to be the open-sky environment whenthe positioning altitude difference is equal to or less than a givenvalue (e.g., 200 m), and determines the reception environment to be themultipath environment when the positioning altitude difference exceedsthe given value.

After the CPU 31 has determined the reception environment, the CPU 31calculates an evaluation point E (indicating positioning accuracy) ofeach satellite set using an evaluation method corresponding to thedetermined reception environment. The higher the evaluation point E, thehigher the positioning accuracy is.

Specifically, when the reception environment is the open-skyenvironment, the CPU 31 calculates the evaluation point E of eachsatellite set using a known method based on the number of GPS satellites(number of satellites) included in the corresponding satellite set, aposition dilution of precision (PDOP) based on the constellation of GPSsatellites, or the signal strength of each GPS satellite signal, forexample.

When the reception environment is the multipath environment, the CPU 31calculates the evaluation point E of each satellite set in the samemanner as in the open-sky environment, and then subtracts a given changeamount ΔE from the calculated evaluation point E. The change amount ΔEis determined as follows. Specifically, the CPU 31 acquires the altitude(preceding altitude) corresponding to the preceding located position asa reference altitude referring to an altitude table.

The term “altitude table” refers to a data table which defines analtitude corresponding to each area (divided area) into which theearth's surface is divided in the shape of a mesh. Each divided area hasa square shape and is defined so that each side of the square extendsalong the latitude direction or the longitude direction. The altitudemay be an arbitrary altitude (e.g., ellipsoidal height or altitude abovesea level).

When the CPU 31 has acquired the altitude (preceding altitude)corresponding to the preceding located position, the CPU 31 calculatesan altitude difference ΔZ (i.e., difference between the altitude of thecandidate present position and the acquired preceding altitude)corresponding to each satellite set. The CPU 31 then calculates thechange amount ΔE based on the calculated altitude difference ΔZ using achange amount calculation equation f. The CPU 31 then subtracts thecalculated change amount ΔE from the evaluation point E of the targetsatellite set.

FIG. 2 is a view showing an example of the change amount calculationequation f. FIG. 2 shows a graph of the change amount calculationequation f in which the horizontal axis indicates the altitudedifference ΔZ and the vertical axis indicates the change amount ΔE. Notethat the change amount ΔE is equal to or larger than zero (ΔE≧0). Asshown in FIG. 2, two calculation equations f1 and f2 are defined as thechange amount calculation equation f. The change amount calculationequation f1 or f2 is selected depending on the calculated positioningaltitude difference. Specifically, the change amount calculationequation f1 is selected when the positioning altitude difference isequal to or smaller than a given value (e.g., 1000 m), and the changeamount calculation equation f2 is selected when the positioning altitudedifference is larger than the given value. The change amount calculationequations f1 and f2 are defined so that the change amount ΔE increasesas the altitude difference ΔZ increases. The slope of the change amountcalculation equation f1 is greater than that of the change amountcalculation equation f2. Specifically, even if the altitude differenceΔZ is identical, the change amount ΔE when the positioning altitudedifference is larger than 1000 m is greater than the change amount ΔEwhen the positioning altitude difference is equal to or less than 1000m.

FIGS. 3A and 3B show examples of experimental results indicating therelationship between the reception environment and a positioning error.FIG. 3A shows experimental results in the multipath environment, andFIG. 3B shows experimental results in the open-sky environment. FIGS. 3Aand 3B show experimental results for a plurality of satellite sets atthe same time in the respective reception environments provided that thepresent position of the portable telephone 1 is known.

FIG. 3A shows a horizontal error and an altitude error of each ofsixteen satellite sets including five or more GPS satellites among sevenGPS satellites (#1 to #7) acquired in the multipath environment, and apositioning altitude difference which is the difference between themaximum value and the minimum value of the altitude errors of all thesatellite sets. The term “horizontal error” refers to the horizontaldifference between the present position of the portable telephone 1calculated based on the GPS satellites of the target satellite set andthe known position of the portable telephone 1. The term “altitudeerror” refers to the vertical difference between the present position ofthe portable telephone 1 calculated based on the GPS satellites of thetarget satellite set and the known position of the portable telephone 1.

FIG. 3B shows the horizontal error and the altitude error of each oftwenty two satellite sets including three or more GPS satellites amongsix GPS satellites (#3, #4, and #8 to #11) acquired in the open-skyenvironment.

When comparing the reception environments shown in FIGS. 3A and 3B, thehorizontal error and the altitude error are larger in the multipathenvironment as compared with the open-sky environment. The horizontalerror and the altitude error differ between the multipath environmentand the open-sky environment by two figures. Specifically, positioningaccuracy is lower in the multipath environment than in the open-skyenvironment. Therefore, the change amount ΔE is subtracted in themultipath environment from the evaluation point E of each satellite setcalculated in the same manner as in the open-sky environment.

As shown in FIG. 3A, the horizontal error and the altitude error of eachsatellite set vary to a large extent in the multipath environment. Thepositioning altitude difference which indicates the degree of variationin altitude error is about 800 m in the multipath environment. In themultipath environment, a signal affected by a multipath is included inthe acquired GPS satellite signals. Since each GPS satellite signal isaffected by a multipath to a different extent, positioning accuracyvaries between the satellite sets.

As shown in FIG. 3B, the horizontal error and the altitude error of eachsatellite set vary to a small extent in the open-sky environment. Thepositioning altitude difference in the open-sky environment is about 2m. In the open-sky environment, since the acquired GPS satellite signalsare not affected by a multipath or the like, positioning accuracy variesto only a small extent between the satellite sets.

In this embodiment, the reception environment is determined based on thevariation in altitude error. Specifically, the reception environment isdetermined to be the multipath environment when the positioning altitudedifference exceeds a given threshold value, and is determined to be theopen-sky environment when the positioning altitude difference is equalto or less than the given threshold value.

In the multipath environment, positioning accuracy differs to a largeextent between the satellite sets due to a large variation in altitudeerror, as described above. In this embodiment, the change amount ΔE ofeach satellite set is determined corresponding to the positioningaccuracy of that satellite set. The positioning accuracy of eachsatellite set is determined based on the altitude difference ΔZ which isthe difference between the altitude (present altitude) of the candidatepresent position of that satellite set and the altitude (precedingaltitude) of the preceding located position. Specifically, thepositioning error is determined to be larger as the altitude differenceΔZ increases, and is determined to be smaller as the altitude differenceΔZ decreases. The change amount ΔE of a satellite set with lowerpositioning accuracy is increased to decrease the evaluation point E ofthat satellite set. Therefore, the change amount calculation equation ffor calculating the change amount ΔZ is defined so that the changeamount ΔE increases as the altitude difference ΔZ increases, as shown inFIG. 2.

In this embodiment, the altitude corresponding to the latitude and thelongitude of the preceding located position is acquired referring to thepredetermined altitude table, and the acquired altitude is determined tobe the preceding altitude as the reference altitude. Note that thealtitude of the preceding located position may be determined to be thepreceding altitude without referring to the altitude table. However,since the located position is not an accurate value, the accuracy ofdetermining positioning accuracy may deteriorate as compared with thecase of using the altitude table.

Moreover, the positioning error of each satellite set differs to a largeextent even in the multipath environment. This is because whether or notthe reception environment is the multipath environment is determinedbased on whether or not the positioning altitude difference exceeds thegiven threshold value. Specifically, the positioning accuracy may belower when the positioning altitude difference exceeds the thresholdvalue to only a small extent than that when the positioning altitudedifference exceeds the threshold value to a large extent in themultipath environment Therefore, two calculation equations f1 and f2corresponding to the positioning altitude difference are defined as thechange amount calculation equation f, as shown in FIG. 2, and thecalculation equation used as the change amount calculation equation f isselected corresponding to the positioning altitude difference.

When the evaluation points E of all satellite sets have been calculatedusing the evaluation method corresponding to the reception environment,a satellite set having the highest evaluation point E is selected fromamong these satellite sets, and the candidate present position Pcorresponding to the selected satellite set is determined to be thepresent located position.

Again referring to FIG. 1, the ROM 32 stores a system program whichcauses the CPU 31 to control each section of the baseband processcircuit section 30 and the RF receiver circuit section 21, and a programand data necessary for the CPU 31 to implement various processesincluding the baseband process. FIG. 4 is a view showing theconfiguration of the ROM 32. As shown in FIG. 4, the ROM 32 stores abaseband process program 321, an altitude table 322, a receptionenvironment determination condition table 323, a calculation equationselection condition table 324, and a change amount calculation equationdata 325.

The altitude table 322 is a data table which defines an altitudecorresponding to each area (divided area) into which the earth's surfaceis divided in the shape of a mesh. FIG. 5 shows an example of the dataconfiguration of the altitude table 322. As shown in FIG. 5, thealtitude table 322 stores an area ID322 a (i.e., identification numberof a divided area), a center position 322 b, and an altitude 322 ccorresponding to each divided area. The latitude and the longitude arestored as the center position 322 b. Each divided area has a squareshape and is defined so that each side of the square extends along thelatitude direction or the longitude direction.

The reception environment determination condition table 323 is a datatable which defines a reception environment determination condition.FIG. 6 shows an example of the data configuration of the receptionenvironment determination condition table 323. As shown in FIG. 6, thereception environment determination condition table 323 stores adetermination condition 323 a corresponding to each receptionenvironment 323 a. A positioning altitude difference condition is storedas the determination condition 323 b.

The calculation equation selection condition table 324 is a data tablewhich defines a selection condition for the change amount calculationequation f. FIG. 7 shows an example of the data configuration of thecalculation equation selection condition table 324. As shown in FIG. 7,the calculation equation selection condition table 324 stores aselection condition 324 b corresponding to each change amountcalculation equation 324 a. A positioning altitude difference conditionis stored as the selection condition 324 b.

The change amount calculation equation data 325 is data which definesthe change amount calculation equation f. For example, functionalequations of the graphs of the change amount calculation equations f1and f2 shown in FIG. 2 are stored as the change amount calculationequation data 325.

The RAM 33 is used as a work area for the CPU 31, and temporarily storesa program and data read from the ROM 32, results of calculationsperformed by the CPU 31 based on various programs, and the like. FIG. 8shows an example of the configuration of the RAM 33. As shown in FIG. 8,the RAM 33 stores satellite position data 331, satellite set data 332,and positioning result data 333.

The satellite set data 331 is data relating to the position of a GPSsatellite acquired by each positioning process. FIG. 9 shows an exampleof the data configuration of the satellite position data 331. As shownin FIG. 9, a satellite position 331 b calculated based on the GPSsatellite signal is stored as the satellite position data 331corresponding to each GPS satellite 331 a acquired.

The satellite set data 332 is data relating to the satellite setselected by each positioning process. FIG. 10 shows an example of thedata configuration of the satellite set data 332. As shown in FIG. 10, aGPS satellite 332 b included in the satellite set, a calculatedcandidate present position 332 c, a number of satellites 332 d, a PDOPvalue 332 e, and an evaluation point 332 f are stored as the satelliteset data 332 corresponding to each selected satellite set 332 a.

The positioning result data 333 is data relating to calculation resultsof each positioning process. FIG. 11 shows an example of the dataconfiguration of the positioning result data 333. As shown in FIG. 11, apositioning time 333 a, a selected satellite set 333 b, and a locatedposition 333 c are stored as the positioning result data 333corresponding to each positioning process from the start of positioning.

Again referring to FIG. 1, the host CPU 40 controls each section of theportable telephone 1 based on various programs such as the systemprogram stored in the ROM 43. Specifically, the host CPU 40 mainlyimplements a telephone call function, and performs a process forimplementing various functions including a navigation function such ascausing the display section 42 to display a navigation screen in whichthe present position of the portable telephone 1 input from the basebandprocess circuit section 30 is plotted on a map.

The operation section 41 is an input device including an operation key,a button switch, and the like. The operation section 41 outputs anoperation signal corresponding to the operation of the user to the hostCPU 40. Various instructions such as a positioning start/finishinstruction are input by operating the operation section 41. The displaysection 42 is a display device such as a liquid crystal display (LCD).The display section 42 displays a display screen (e.g., navigationscreen and time information) based on a display signal input from thehost CPU 40.

The ROM 43 stores a system program which causes the host CPU 40 tocontrol the portable telephone 1, a program and data necessary forimplementing a navigation function, and the like. The RAM 44 is used asa work area for the host CPU 40. The RAM 44 temporarily stores a programand data read from the ROM 43, data input from the operation section 41,results of calculations performed by the host CPU 40 based on variousprograms, and the like.

The portable telephone antenna 50 is an antenna which transmits andreceives a portable telephone radio signal between the portabletelephone 1 and a radio base station installed by a communicationservice provider of the portable telephone 1. The portable telephonewireless communication circuit section 60 is a portable telephonecommunication circuit section which includes an RF conversion circuit, abaseband process circuit, and the like, and transmits and receives aradio signal under control of the host CPU 40.

Process Flow

FIG. 12 is a flowchart illustrative of the flow of the baseband process.The baseband process is implemented by causing the CPU 31 to execute thebaseband process program 321. A digital IF signal obtained bydown-converting an RF signal received by the GPS antenna 10 into an IFsignal by the RF receiver circuit section 21 is input to the basebandprocess circuit section 30 before the baseband process.

As shown in FIG. 12, the CPU 31 calculates satellite information(including the position) relating to each GPS satellite based on theacquired GPS satellite signal (step A1). The CPU 31 selects satellitesets made up of four or more GPS satellites based on the acquiredsatellite signals (step A3), and performs a loop A process on eachselected satellite set.

In the loop A, the CPU 31 calculates the pseudo-range between theportable telephone 1 and each GPS satellite included in the targetsatellite set based on the position of each GPS satellite, for example,and calculates the candidate present position P by performingpositioning calculations using a least-square method (step A5). The CPU31 then performs an evaluation process using the number of satellitesand the PDOP value of the target satellite set and the like to calculatethe evaluation point E for the candidate present position Pcorresponding to the target satellite set (step A7). The loop A is thusperformed.

When the CPU 31 has performed the loop A process on all satellite sets,the CPU 31 extracts the maximum value (maximum altitude) and the minimumvalue (minimum altitude) of the altitudes of the calculated candidatepresent positions P corresponding to the satellite sets, and calculatesthe positioning altitude difference (i.e., difference between themaximum value (maximum altitude) and the minimum value (minimumaltitude)) (step A9). The CPU 31 determines the reception environmentbased on the calculated positioning altitude difference using thereception environment determination condition table 324 (step A11).

When the CPU 31 has determined that the reception environment is themultipath environment (step A13: “Multipath”), the CPU 31 performs anevaluation point change process. Specifically, the CPU 31 selects thechange amount calculation equation f based on the calculated positioningaltitude difference using the calculation equation selection conditiontable 324 (step A15). The CPU 31 acquires the altitude (precedingaltitude) corresponding to the preceding located position referring tothe altitude table 322 (step A17).

The CPU 31 then performs a loop B process on each satellite set. In theloop B, the CPU 31 calculates the altitude difference ΔZ which is thedifference between the altitude of the candidate present position Pcorresponding to the target satellite set and the acquired precedingaltitude (step A19). The CPU 31 changes the change amount ΔE based onthe calculated altitude difference ΔZ according to the selected changeamount calculation equation f (step A21). The CPU 31 subtracts thecalculated change amount ΔE from the evaluation point E of the targetsatellite set (step A23). The loop B is thus performed. The evaluationpoint change process is thus completed.

The CPU 31 then selects a satellite set with the highest evaluationpoint E from the satellite sets, and determines the candidate presentposition P corresponding to the selected satellite set to be the presentlocated position of the portable telephone 1 (step A25). The CPU 31 thendetermines whether or not to finish positioning. When the CPU 31 hasdetermined to continue positioning (step A27: NO), the CPU 31 returns tothe step A1 and performs the next positioning. When the CPU 31 hasdetermined to finish positioning (step A27: YES), the CPU 31 finishesthe baseband process.

Modification

Embodiments to which the invention may be applied are not limited to theabove-described embodiments. Various modifications and variations may bemade without departing from the spirit and scope of the invention.

(A) Determination of Reception Environment

In the above embodiments, the reception environment is determined basedon the positioning altitude difference (i.e., difference between themaximum value (maximum altitude) and the minimum value (minimumaltitude) of the altitude) as a dispersion which indicates an index thatindicates the degree of distribution (variation) of the altitudes of thepresent positions P corresponding to the satellite sets. Note that thereception environment may be determined based on another dispersion suchas a variance or a standard deviation.

(B) Reception Environment

In the above embodiments, the reception environment is classified as themultipath environment and the open-sky environment. Note that thereception environment may be classified as three or more environmentsincluding an indoor environment, for example. In this case, thedetermination condition is appropriately set for each receptionenvironment.

(C) Change Amount Calculation Equation f

In the above embodiments, the calculation equation f1 or f2 is used asthe change amount calculation equation f. Note that three or morecalculation equations f1, f2, f3, . . . may be used. In this case, theselection condition is appropriately set for each calculation equation.

(D) Host CPU

Some or all of the processes performed by the CPU 31 of the basebandprocess circuit section 30 may be performed by the host CPU 40 by meansof software.

(E) Positioning Device

The above embodiments have been described taking an example of aportable telephone which is an electronic instrument including apositioning device. Note that the invention may also be applied to otherelectronic instruments such as a portable navigation system, a carnavigation system, a personal digital assistant (PDA), and a wristwatch.

(F) Satellite Positioning System

The above embodiments have been described taking an example utilizingthe GPS. Note that the invention may also be applied to other satellitepositioning systems such as the global navigation satellite system(GLONASS).

(G) Storage Medium

A configuration may be employed in which the baseband process program321 is recorded on a recording medium such as a CD-ROM and installed inan electronic instrument such as a portable telephone.

Although only some embodiments of the invention have been described indetail above, those skilled in the art would readily appreciate thatmany modifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of the invention.Accordingly, such modifications are intended to be included within thescope of the invention.

1. A positioning method when receiving satellite signals transmittedfrom positioning satellites and repeating present position positioningcalculations based on the received satellite signals, the positioningmethod comprising: selecting satellite sets based on the receivedsatellite signals, each of the satellite sets being a combination ofsatellites used for present positioning calculations; calculatingcandidate present positions including a present position altitudecorresponding to the respective satellite sets using the satellitesignals from the satellites included in the respective satellite sets;determining a reception environment based on a dispersion of the presentposition altitudes of the respective satellite sets; evaluating thecandidate present positions corresponding to the respective satellitesets using an evaluation method corresponding to the receptionenvironment; and selecting a candidate present position from thecandidate present positions corresponding to the respective satellitesets based on the evaluation results, and determining the selectedcandidate present position to be a present located position.
 2. Thepositioning method as defined in claim 1, the determining of thereception environment including determining the dispersion based on adifference between a maximum altitude and a minimum altitude of thepresent position altitudes of the respective satellite sets.
 3. Thepositioning method as defined in claim 1, the determining of thereception environment including determining a reception environment froma plurality of reception environments including a multipath environment;and the evaluating of the candidate present positions includingadjusting the evaluation results of the candidate present positionscorresponding to the respective satellite sets based on a differencebetween the present position altitude of a target satellite set amongthe satellite sets and a given reference altitude, and increasing anadjustment amount when the reception environment has been determined tobe the multipath environment as compared with a case where the receptionenvironment has been determined to be a reception environment other thanthe multipath environment.
 4. The positioning method as defined in claim3, the evaluating of the candidate present positions includingevaluating the candidate present positions using an altitude of alocated position determined by preceding positioning calculations as thegiven reference altitude.
 5. The positioning method as defined in claim4, the evaluating of the candidate present positions including acquiringan altitude corresponding to the located position determined by thepreceding positioning calculations from given altimeter data thatdefines altitudes of each area, and evaluating the candidate presentpositions using the acquired altitude as the given reference altitude.6. A positioning method when receiving satellite signals transmittedfrom positioning satellites and repeating present position positioningcalculations based on the received satellite signals, the positioningmethod comprising: selecting satellite sets based on the satellitesignals, each of the satellite sets being a combination of satellitesused for present positioning calculations; calculating candidate presentpositions including a present position altitude corresponding to therespective satellite sets using the satellite signals from thesatellites included in the respective satellite sets; evaluating thecandidate present positions corresponding to the respective satellitesets using a given evaluation method that adjusts an evaluation resultbased on a difference between the present position altitude of a targetsatellite set among the satellite sets and a given reference altitude;and selecting a candidate present position from the candidate presentpositions corresponding to the respective satellite sets based on theevaluation results, and determining the selected candidate presentposition to be a present located position.
 7. A positioning methodcomprising: selecting satellite sets, each of the satellite sets being acombination of satellites used for present positioning calculations;calculating candidate present positions including a present positionaltitude corresponding to the respective satellite sets using satellitesignals from the satellites included in the respective satellite sets;determining a reception environment based on a dispersion of the presentposition altitudes of the respective satellite sets; evaluating thecandidate present positions corresponding to the respective satellitesets using an evaluation method corresponding to the receptionenvironment; and selecting a candidate present position from thecandidate present positions corresponding to the respective satellitesets based on the evaluation results, and determining the selectedcandidate present position to be a present located position.